UI : Title bar has a wrong position

Hello,

I make a Xamarin.Forms app, and i've encountered a UI problem.
The Title Bar has a wrong position in my page and i don't understand why ... it's probably very simple ...
If somebody knows and can help me, it'll be very nice.

Here's the XAML code and a screenshot of the result on iOS.

Thanks,
Simon

<?xml version="1.0" encoding="UTF-8"?>
<ContentPage 
    xmlns="http://xamarin.com/schemas/2014/forms" 
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
    x:Class="Views.TaskDetailsPage" Title="Détails">

    <ContentPage.Content>
        <StackLayout>
            ...
        </StackLayout>
    </ContentPage.Content>
</ContentPage>